In recent years, various vehicle control systems, such as ABS (anti-lock brake system), TCS (traction control system), and VSC (vehicle stability control) have been developed, in order to secure traveling stability and traveling safety of an automobile. In order to control the vehicle control systems, it is required to accurately detect a situation (for example, rotational speed, a load, temperature, or the like) of a traveling tire.
In view of these circumstances, for example Patent Document 1 discloses a technique to obtain force (ground contact load) acting on a tire by providing a load sensor in a rolling bearing unit used for a suspension device rotatably supporting a vehicle wheel. According to this technique, the load acting on the traveling tire can be obtained so as to control an attitude of an automobile.
However, as described above, when the load acting on the tire is obtained using the load sensor provided in the rolling bearing unit, the load detected by the load sensor provided in the rolling bearing unit does not match the load actually acting on the tire in a state where a brake (braking force) is acting. This is because, in the state where the brake is acting, there are two paths, through which the load acting on the tire is transmitted to a vehicle body. One is “Tire ->Rolling Bearing Unit ->Vehicle Body”, and the other is “Tire ->Brake Device ->Vehicle Body”. The load sensor provided in the rolling bearing unit can only detect the load transmitted to the former path.
On the other hand, if acceleration of the tire (translational acceleration in an upper-lower direction) can be measured, the load acting on the tire can be obtained regardless of an operating state of the brake. However, in a case where an acceleration sensor is directly provided on the tire, it is necessary to re-attach the acceleration sensor at the time of periodically required tire replacement, resulting in a cost increase. There has been known that members (hereinafter, referred to as “unsprung members”) such as the vehicle wheel (the tire and a wheel), the rolling bearing unit, a brake part, a knuckle, and a suspension upright, which are provided below springs configuring suspension of the automobile, move together with the tire in the upper-lower direction. The inventors of the present invention have found that among the unsprung members, as long as the translational acceleration acting on the rolling bearing unit, unlike the tire which requires periodic replacement, can be measured, the load acting on the tire can be obtained regardless of the operating state of the brake, while suppressing the cost increase.